It is widely believed that stars form from collapsing interstellar clouds. However, molecular clouds typically contain of the order of 103 solar masses. Thus a mechanism is required that allows a collapsing cloud to fragment into a number of collapsing stellar sized sub-condensations. The early work was based on a virial theorem approach—defining a critical mass a cloud must exceed in order for its gravitational force to overcome the resistive thermal, rotational and magnetic forces, thus allowing the cloud to collapse. This critical mass is analogous to the Jeans mass for non-rotating, non-magnetic clouds. It is thought that a cloud containing several critical masses may collapse into several sub-condensations. Further, it is thought that the galactic magnetic field will cause the critical mass to decrease as the cloud collapses, allowing the cloud to fragment into a number of sub-condensations. The critical mass decreases as the cloud flattens down the field lines (since Mcrit ∝ B3/ϱ2 and B ∝ ϱk, K < ⅔ for non-isotropic collapse).